The response of a bulk-acoustic wave resonator (FBARs, SMRs, HBARs, etc.) exhibits long term drift in its characteristics, particularly in frequency. This time-dependent long-term change is known as drift of the resonator. The drift is caused by both intrinsic and extrinsic factors and the intrinsic instability is often called as aging of the resonator. Aging occurs even when external environmental factors are kept constant. An example of an SMR structure from the prior art is illustrated in FIG. 1.
In the literature, Walls and Vig, “Fundamental Limits on the Frequency Stabilities of Crystal Oscillators,” IEEE Transactions On Ultrasonics, Ferroelectrics, And Frequency Control, 42(4):576-589, July 1995 (referred to herein as Walls and Vig, 1995) and Vig and Meeker, “The Aging of Bulk Acoustic Wave Resonators, Filters, and Oscillators,” Proc. 45th Ann. Symp. Frequency Control, IEEE Cat. No. 91 CH2965-2, pp. 77-101, (1991) (referred to herein as Vig and Meeker 1991) present a taxonomy of mechanisms that cause aging in resonators and oscillators. These mechanisms include mass transfer to or from the resonator's surfaces due to deposition or removal of contaminants, stress relief in the mounting structure of the crystal, changes in the electrodes, leaks in the package, and changes in the piezoelectric material. Other mechanisms include external environmental effects like temperature and stress cycling (hysteresis) and inertial effects.
In general, previous attempts in making a resonator stable against environmental effects and aging have focused on frequency stability. This effort has focused on packaging and mounting structure design.
Usually, packaging of the resonator has been the primary method of protecting it against aging that is caused by contamination and leaks. Also, packaging partially insulates the resonator from external environmental effects.
Quartz resonators have traditionally been packaged within containers to protect them from certain aging phenomena. Many examples exist in the prior art. For example, see U.S. Pat. No. 5,640,746 entitled “Method of Hermetically Encapsulating a Crystal Oscillator Using a Thermoplastic Shell” to Knecht et al. that issued on Jun. 24, 1997.
Micromachined thin film resonators are packaged using wafer-scale or device-scale encapsulation techniques. Many examples exist in the prior art. Micromachined thin film resonators like silicon resonators and thin-film bulk acoustic wave resonators (FBARs) use micromechanical support structures such as posts and suspensions. These structures are also designed to minimize the transfer of stress, including temperature-induced stress to the crystal resonator. For example, see Kim et al. “Frequency stability of wafer-scale film encapsulated silicon-based MEMS resonators,” Sensors and Actuators A 136 (2007) 125-131. Also see U.S. Pat. No. 7,153,717 entitled “Encapsulation of MEMS Devices Using Pillar-Supported Caps” to Carley et al. that issued on Dec. 26, 2006.
The current methods of packaging are either high profile (the case with quartz), which makes them difficult to integrate in a product; or they encapsulate thin-film structures like released inertial resonators or FBARs that are susceptible to other forms of instability such as acceleration or shock.
The mounting structure is another location for possible aging. Stress introduced by packaging and transmitted to the crystal causes long-term frequency aging.
Quartz resonators have been mounted via support legs before being sealed under a cap. The support structure is carefully designed to minimize stress transfer (hence aging) to the crystal. Many examples exist in the prior art. For example, see U.S. Pat. No. 4,642,510 entitled “Mount for quartz crystal oscillator device” to Yamashita that issued on Feb. 10, 1987. See also U.S. Pat. No. 5,030,875 entitled “Sacrificial Quartz Crystal Mount” to Knecht, that issued on Jul. 9, 1991. Levitating the crystal using electrostatic levitation so that aging effects related to a mechanical mounting structure are minimized has also been suggested. See Wall and Vig, 1995.
The current methods of mounting of the resonator are susceptible to inertial and thermal fatigue, hence aging. None of these approaches, however, address the protection of the crystal and/or electrode material itself. The current approaches do not protect the crystal or electrode material from environmental effects, including aging.
The research focus to date on frequency stability is appropriate for certain applications. However, a more general focus on the entire behavior of the resonator around the primary resonance, in both frequency (f) and over time is desired.